The present invention concerns a process for the production of semiprocessed non oriented grain electrical sheet with high magnetic permeability and low magnetic losses. More precisely it concerns a steel with a low S, N and C content characterized by careful control of chemical composition and treatment via an appropriate thermomechanical cycle during manufacture. Non oriented grain sheet is, of course, marketed in "semiprocessed" and "processed form, the former requiring successive heat treatment by the user.
In both cases the sheet is used in the cores of electrical machines, in low-power transformers, in relays and in starters for lights.
If constructors so require, namely when it is necessary to produce high-output motors, such as for instance in the case of sealed units for refrigerators, the following solutions are commonly selected: increase in size of core to reduce magnetic induction, reduction in sheet thickness, and increase in Si content. In all cases manufacturing costs are markedly higher.
The alternative solution is to produce sheet that unites the characteristic of low magnetic losses with that of high magnetic permeability, thus ensuring more contained dissipation of energy both in the core and the windings.
To obtain this type of sheet, action must be taken on the variables that control magnetic permeability and total magnetic losses, and particularly losses due to static hysteresis which, of course, depend mainly on the inclusions content and grain size. The inclusions commonly present are oxides, sulphides and nitrides. The oxygen content is normally limited by the addition of dexoidants or by vacuum carbodeoxidation. The sulphur is reduced by the addition of desulphurizing elements, while the adverse influence of nitrogen, which is inevitably present, is limited by high-temperature precipitation as AlN; the amount of Al used does not generally exceed 0.5%.
Regarding grain growth capable of improving magnetic permeability and magnetic losses, it should be recalled that this can be attained either by high-temperature annealing (800 .degree. C. or more) of the cold-rolled sheet, or by the joint action of critical cold rolling of the recrystallized sheet with reduction of area of about 6-8%, and subsequent decarburizing annealing performed as per Euronorm 165/81.
In both cases the growth of crystalline grain is accompanied by evolution of the corresponding texture towards magnetically less favourable components, thus limiting the benefits obtained. The normal production process for non oriented grain sheet includes heating the slab to about 1250 .degree. C., hot rolling to strip about 2 mm thick, sand-blasting, pickling, cold rolling, recrystallization annealing, cold rolling with reduction of area of about 5-8% and subsequent decarburizing annealing conducted by the user of the cut product.
Surprisingly, it has now been found that with the combination of careful refining of the liquid steel, appropriate chemical composition, a slab-to-sheet working process as per the invention, and annealing of the ensuing hot strip at a suitable temperature which depends on the Si content, it is possible to obtain non oriented grain electrical sheet or strip with higher magnetic permeability and lower magnetic losses than can be obtained with known methods on sheet of the same thickness and Si content.